A piezoelectric acoustic element using a piezoelectric element as a vibration source has various advantages, such as being compact, lightweight, power-thrifty, and does not leak magnetic flux, and therefore is expected to be used as an acoustic part of a portable terminal device. In particular, since the mounting volume can be significantly reduced in comparison with the conventional electromagnetic acoustic element, the piezoelectric acoustic element is considered as one critical technique for further reducing size of portable telephones.
However, the sound source of the piezoelectric acoustic element is a vibration plate that bends in accordance with the deformation of the piezoelectric element. Therefore, in order to ensure the sound pressure level that is required to reproduce sounds, the vibration plate must be bent above some level and a large vibration plate is required. For example, in the conventional piezoelectric acoustic element, a vibration plate of 20 [mm] in diameter is required to obtain the sound pressure of 90 [dB] when voltage of 1 [V] is applied to the piezoelectric element, and therefore it causes the piezoelectric acoustic element to lose advantages such as compact and lightweight.
Next, the frequency characteristics of the conventional piezoelectric acoustic element are described. The piezoelectric acoustic element has the following problems.
(1) a basic resonant frequency appears in the audible range,
(2) a frequency characteristic is included so as to generate an unusual sound pressure near the resonant frequency, and
(3) since ceramic used as a piezoelectric material for the piezoelectric element has high stiffness, the basic resonant frequency becomes higher and no sufficient sound pressure can be obtained in a low frequency range.
In order to reproduce the original sound faithfully, the basic resonant frequency must be adjusted at 500 [Hz] or less. So, Japanese Patent Laid-Open No. 4-22300 discloses the technique in which the carbon plate (expansion graphite plate) is used as the vibration plate to improve the frequency characteristic. Also, it is known that the frequency characteristic is improved to some extent by forming the vibration plate into an ellipse.
Next, the frequency-sound pressure characteristic of the conventional piezoelectric acoustic element is described. The conventional piezoelectric acoustic element uses the piezoelectric element as the vibration source, as described above. As the piezoelectric material of the piezoelectric element, ceramic materials and the like with a small loss of mechanical energy during elastic vibration are usually used. Therefore, very high sound pressure can be obtained near the resonance point, however, the irregular frequency-sound pressure characteristic with a large amplitude change will occur in the frequency range except the resonance point. When the amplitude change of the frequency-sound pressure characteristic is large, only sound at a specific frequency is emphasized, and therefore sound quality will deteriorate. So, Japanese Utility Model Laid-Open No. 63-81495 discloses a technique in which a piezoelectric vibrator is buried in flexible foam to flatten the frequency-sound pressure characteristic. Also, Japanese Patent Laid-Open No. 58-8000 discloses a technique that flattens the frequency-sound pressure characteristic by supporting the outer edge of a thin acoustic element by foam formed with an adhesive layer on the surface thereof.
[Patent Document 1] Japanese Patent Laid-Open No. 4-22300
[Patent Document 2] Japanese Utility Model Laid-Open No. 63-81495
[Patent Document 3] Japanese Patent Laid-Open No. 58-8000